In the past, air traffic control radar beacon system ("ATCRBS"), transponder communication systems, utilized Mode C or Mode A protocols in the communication/transmission of altitude and identify or only identity information, respectively, between aircraft and a beacon interrogator secondary surveillance radar (SSR) system. In both the Mode A and Mode C systems, when transmitting information, a secondary surveillance radar would sequentially transmit interrogation signals to aircraft in the area to request information from the aircraft. This signal transmitted by the SSR would contain three (3) pulses with the first pulse and third pulse being separated by a predetermined width wherein these pulses are transmitted at a specific frequency or control tone. The first and third pulses are transmitted from a radar dish having directional characteristics, i.e., 3.degree. beam width. The second pulse is a side lobe suppression signal transmitted from an omni directional antenna co-located at the directional dish. The interrogated object will only recognize this signal if the amplitude of the second pulse is less than the first or third pulse. The time interval between the first and third pulses defines what information the interrogator is requesting, i.e., eight (8) microseconds for identification and twenty-one (21) microseconds for altitude. Upon receipt of this signal, the aircraft would develop a reply signal to supply the requested information consisting of identification and/or altitude location. The SSR will process this information, together with time of arrival range information, to develop a measurement of position for each responding aircraft. The Mode C and Mode A systems are unable to relay information or messages from the SSR to the interrogated aircraft or allow the aircraft to transmit any message other than identification and altitude. The Mode C and Mode A systems also encounter a large amount of interference and garble because Mode C or Mode A interrogation from an ATCRBS beacon interrogation is special, i.e., all aircraft transponds within the main beam of the interrogating SSR reply. This means that 25-45 interrogation and replies are transmitted into the radio frequency environment. This results in proliferation of radio frequency transmissions which can result in a significant amount of interference or garble and a corresponding loss of integrity in the developed track data.
More specifically, an average Air Traffic Control Radar Beacon System (ATCRBS) interrogator sends out 250 to 450 interrogations per second per radio frequency. In a ten second period, an ATCRBS radar will dwell on a specific aircraft for about 100 milliseconds which implies 25 to 45 replies being received by the ATCRBS when only one or two replies are really needed. The result is a multitude of overcapped signals, distortion, and degradation to the ability of the ground controller to carry out surveillance on the local air traffic.
In recognizing these and other deficiencies in the ATCRBS, the Mode Select (Mode S or Discrete Beacon Address System, DBAS) was developed to allow the active transmission of information by a SSR or an aircraft which substantially reduced transmission interference or garble. To realize this purpose, the Mode S sensors were developed to interrogate the targets discretely, not spacially like the ATCRBS. The Mode S sensor transmits a specific I.D. code, i.e., a code specific to the aircraft from which the sensor wants a reply; again, only once or twice. The transmission from the sensor occurs only once or twice, and the aircraft's Mode S transponder will reply. However, Mode S was also developed to function within a Mode A or Mode C environment, i.e., the Mode S preamble can function in the Mode A or Mode C environment. Thus, the Mode S detection circuits will allow full surveillance in an integrated ATCRBS/Mode S environment.
The Mode S sensor produces a tag if aircraft are in the surveillance area by using two techniques, thereby enabling the discrete addressing of these aircraft. One technique is a Mode S SQUITTER performed by the Mode S transponder and the other technique is a Mode S ALL CALL performed by the sensor. In the Mode S ALL CALL, the Mode S emits an ATCRBS like spacial identify interrogation signal which elicits a transponder reply transmission of discrete identification. In the Mode S SQUITTER, the Mode S transponder pseudo-randomly transmits a specific address code, unique to the aircraft to be interrogated, once per second within a range of 200 milliseconds.
To achieve discrete interrogation and elicit or a transponder reply transmission, the Mode S system relies on a preamble having two (2) pulses separated using a predetermined width and are similar in format and modulation to ATCRBS interrogation pulses described above. These pulses are also transmitted by a specific frequency or control tone. Following the Mode S interrogation precycle, the sensor will transmit a differentiated phase shift keyed message of 56 or 112 bit length. To distinguish the preamble of the Mode S transmission from the pulse signals of the Mode C and Mode A transmissions, the Mode S system also transmits a sync phase reversal signal just prior to the transmission of the differential phase-shift keyed data. This transmitted sync phase reversal signal must be transponder by the Mode S transponder within a certain period or time window immediately following the detection of the preamble. If this sync phase reversal signal is received within this time window, the receiver will start processing the subsequent differential phase-shift data being transmitted by the SSR. If the sync phase reversal signal is not received, the transponder may generate a Mode A or Mode C reply.
To implement the receipt of a Mode S signal, analog systems including well known Costa's phase lock loop demodulators, have been developed which are capable of detecting the preamble and sync phase reversal signal while also being capable of demodulating the differential phase-shift keyed data. However, these analog systems require a relatively significant amount of time to acquire Mode S uplink or interrogation signal. Also, the state of the art Mode S data demodulators are not capable of efficiently filtering out the noise associated with the Mode S uplink or interrogation signal in an integrated Mode S/ATCRBS RF environment. Lastly, the analog Mode S demodulator would require a relatively large amount of space in the aircraft, necessitating considerable weight, volume, power comsumption and cost.